Process for improving fatigue life in spring-cushioned shoes

ABSTRACT

A process for reducing heat transfer within a spring in a spring cushioned shoe, said process comprises the steps of applying a residual compressive stress to said spring, for example by shot peening, and then mounting the spring between an inner sole and an outer sole of the shoe.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 USC Section 119, this application claims the benefit ofpriority from Provisional Application Ser. No. 60/355,485 with a filingdate of Feb. 8, 2002, and Non-provisional Application Ser. No.10/358,514 with a filing date of Feb. 5, 2003.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to the field of shoes, and more particularly tospring cushioned shoes.

In most running, walking, and jumping activities, the return forceresulting from foot strikes causes great shock to the body. Repeatedfoot strikes stress joints and bones, and can lead to injuries to thelower back and the rotating joints of the legs.

To minimize injury to the body resulting from repeated foot strikes, andalso to 10 improve athletic performance, shoe engineers have addedsprings to the soles of shoes. The springs in spring-cushioned shoes aredesigned to reduce shock to the body during a foot strike, and also torecover and return impact energy to the user. Various spring-cushionedshoe designs are described in U.S. Pat. No. 6,282,814 to Krafsur et al.,pending U.S. patent application Ser. No. 09/982,520 to LeVert et al.,and U.S. Pat. No. 5,743,028 to Lombardino, all of which are incorporatedherein by reference.

Shoes incorporating metal springs, however, have had two distinctdisadvantages: (1) they are heavier than traditional shoes; and (2) thesprings often set or fail prematurely. Prior solutions to these twodisadvantages are conflicting. Making shoes lighter by, e.g., reducingthe size of the spring's coil, causes the springs to fail earlier, andusing sturdier springs makes the shoes heavier.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to produce a spring-cushioned shoein which the springs do not fail prematurely, but the shoe is notundesirably heavy.

Another object of the present invention is to reduce the transfer ofheat to the heat-sensitive portions of the shoe that are in contact withthe springs.

As used herein, a material's “tensile strength” is the stress point atwhich a material will either break or deform beyond usefulness.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and in the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWING

The above-mentioned features of the invention will become more clearlyunderstood from the following detailed description of the invention readtogether with the drawing in which:

The FIGURE is a cross-sectional view of a wire experiencing a load.

DETAILED DESCRIPTION OF THE INVENTION

In most spring-cushioned shoes, metal compression springs are embeddedin the sole of the shoe to provide cushioning and energy return. Thesprings are generally formed from one or more wires coiled into aparticular shape. The springs can be, e.g., disk springs, cone springs,Belleville springs, or, as described recently in U.S. Pat. No.6,282,814, wave springs, such as multi-turn crest-to-crest wave springs.

When a spring is placed under a load, the metal wire-forming the springis subject to certain stresses. Consider, for example, a rectangularmetal wire subject to a bending stress. Referring to FIG. 1, a wire 10is subject to a load (force arrows F) that bends the wire. Under load F,an outer section 12 of the wire 10 is stretched, while an inner section14 is compressed. The outer section 12 is therefore subject to tension,or tensile stress, while the inner section 14 is subject to compressivestress. If the wire fails (e.g., cracks), it will fail on the tensilestressed side 12, not the compressed side 14.

When a person wearing a spring-cushioned shoe stands, walks, or runs inthe shoe, the springs in the shoe are subject to loading, which stressesthe metal in the spring. Some portions of the spring are subject totensile stress, or “tensile loading,” while others are subject tocompressive stress. As discussed above, tensile stress, not compressivestress, potentially causes failure. Thus, the portions of the springsubject to failure are the portions that experience tensile loading.Over time, cyclical tensile loading fatigues the metal in the spring,and can cause the spring to set (i.e., fail to return to its originalstate after removal of a load) or the metal to crack and fail.

Spring failure from cyclical tensile loading can be delayed or avoidedby, e.g., increasing the thickness of the spring wire. As discussedabove, however, this makes the spring-cushioned shoe heavier, which isundesirable.

The present invention relates to pre-treating springs used inspring-cushioned shoes to impart a residual compressive stress by, e.g.,a process known as “shot peening.” The shot peening process imparts apermanent compressive stress on the spring, which counters the tensileloading that results from standing, walking, or running in the shoe. Theshot peening process therefore enhances the spring's ability towithstand cyclical tensile loading, and increases the useful life of aspring in a spring-cushioned shoe, without increasing the shoe's weight.

The conditioning of the surface 16 of the spring 10 by the process ofcold working or hammering the surface 16 with small spheres of steel,ceramic or glass media propelled against the surface 16 of the spring 10puts the upper layers of the material into compression and helps toprevent failure in the material as stated hereinabove. The physicalchange in the volume of the spring 10 from the compression, thoughsmall, causes a significant positive change in the magnitude of theelectrical conductivity of the spring material near its surface 16. Itis known that the resistance of a material is inversely related to itselectrical conductivity. Because both heat energy and electrical energyare carried by free electrons in a metal, a good electrical conductor isgenerally a good heat conductor. Conversely, a poor electrical conductoris generally a poor heat conductor.

Mechanical energy is converted into thermal energy during the constantflexing of the spring 10 when it is mounted in a shoe sole. Therepetitive flexing and expansion of the spring 10 causes the temperatureof the spring 10 to rise above the ambient temperature. In accordancewith the present invention, shot peening is used to increase the surfacecompressive stress of the spring 10 and thereby reduce the heatconductivity of the spring 10. Accordingly, there is a reduction in theamount of thermal energy conducted through the spring 10 to the moretemperature sensitive materials of the shoe that contact the spring 10.Instead, the thermal energy developed in the spring 10 is more uniformlydispersed through the shoe by the convection heat transfer of the air orother fluid media surrounding the spring 10.

Shot Peening: In shot peening, a substrate surface, usually metal, isbombarded with small media called “shot.” Shot pieces are usuallyspherical in shape, and harder than the substrate they strike. When apiece of shot strikes the substrate, it creates a small dimple in thesurface of the substrate. The metal grains displaced by the shot strikeimpart a compressive force onto the sides of the dimple, trying torestore the surface of the metal to its original shape. If the shotpeening process covers the surface of the substrate with overlappingdimples, then the entire surface will have a uniform, permanent,residual compressive stress at and near its surface. The residualcompressive stress imparted to the surface of the substrate by shotpeening is generally about half the tensile strength of the substratematerial. Just below the surface the residual compressive stressimparted is greater, e.g., about 60% of the tensile strength. (See MetalImprovement Company, Inc. Shot Peening Applications 6-7 (8th ed. 2001).

Wave Spring Embodiment: In one embodiment, the crest-to-crest,multi-turn wave springs used in the spring- cushioned shoes of U.S. Pat.No. 6,282,814 are pre-treated with shot peening. Prior to placing thesprings within the heel and ball vacuities of the shoe sole, bothsprings are shot peened for, e.g., about 10-15 minutes with 0.023 inchdiameter spherical shot. All exposed surfaces of the spring, includinginner and outer surfaces, are bombarded with shot. After the 10-15minutes of shot peening, the entire exposed surface is covered withdimples, such that the dimples overlap. Each dimple is, e.g., up to0.0004152 square inches in area, and there are, e.g., at least 2400dimples per square inch of surface area. The springs are made from,e.g., 1075 carbon steel or 17-7 PH stainless steel with a Rockwellhardness of 53. The shot has a hardness greater than the hardness of thesprings. After the shot peening process is complete, the springs areinserted into the heel and ball vacuities of the shoe, as shown in U.S.Pat. No. 6,282,814, which is incorporated herein.

The shot peening process imparts a permanent compressive stress to thesurface of the spring equal to, e.g., about 50% of the spring's tensilestrength. Just below the surface, the dimples impart maximum compressivestresses of up to, e.g., about 60% of the tensile strength. Thisresidual compressive stress allows the spring to more easily withstandtensile loading, and therefore improves the fatigue life of the spring,and the useful life of the spring-cushioned shoe.

Comparison Example: In this Example, we compare two spring-cushionedshoes, one with shot peened springs and one with non-shot peenedsprings. We demonstrate that the shot peened springs can be made thinnerand lighter, and still achieve a satisfactory fatigue life.

Both spring-cushioned shoes have substantially the structure shown inU.S. Pat. No. 6,282,814. The springs are crest-to-crest, multi-turn wavesprings made from flat wire steel having a tensile strength of 211 ksi(where 1 ksi=1000 psi). The outer diameter (O.D.) of the wire is 2.5inches, and the inner diameter (I.D.) is 2.0 inches. The spring coil has3.5 waves per turn. Below, we demonstrate that the shot-peened springcan have a wire thickness about 31% less than the non-shot peenedspring, and will therefore be 31% lighter.

Consider first the non-shot peened spring, which has no residualcompressive stress. According to the Engineering and Parts Catalog ofSmalley Steel Ring Company (a manufacturer of wave springs), acrest-to-crest multi-turn wave spring experiences a bending stress, ortensile stress, according to the following equation: (1)S=(3πPD_(m))÷(4bt²N²) where S is tensile stress in psi, P is load inpounds, Dm is the spring's mean diameter [(O.D+I.D.)÷2] in inches, t isthe thickness of the wire in inches, and N is the number of waves perturn. According to the Smalley Catalog, for the wave spring to endureone million load cycles without failure, the spring should be operatedin stress range no greater than 50% of the tensile strength. If the shoeis worn by an average-sized man of 160 pounds, then an average cycle(e.g., a step) will impart a load P of about 160 pounds.

If we let S=105,500 psi and P=160 in equation (1) and solve for thespring thickness t, we see that t must be at least 0.051 inches in thenon-shot peened spring to last one million cycles.

In the shot-peened spring (shot peened as described above), the springhas a residual compressive stress near its surface equal to 60% of thetensile strength, or 126.6 ksi. Thus, in equation (1), we letS=105,500÷126,600=232,100 psi. Solving for t, we find that the thicknesscan be 0.035 inches, and still withstand one million cycles.

By using a shot peened spring, therefore, the thickness of the springcan be reduced by about 31%, which translates to a 31% reduction in theweight of the spring. For the wave springs described in U.S. Pat. No.6,282,814, shot peening allows the weight of each spring to be reducedfrom, e.g., about 2.0 ounces to approximately 1.4 ounces. Since eachshoe in this embodiment includes two wave springs, shot peening reducesthe weight of each shoe by, e.g., about 1.2 ounces.

In fact, however, shot peening allows the weight to be reduced even morethan 31% for each spring. If the springs are shot peened, it is possibleto use a considerably more brittle (i.e., less ductile) metal material,without fear of early failure. For example, instead of using a metalwith a tensile strength of 211 ksi, it is possible to use harder metal,with a tensile strength of about 275 ksi. Using a shot peened 275 ksimetal, the spring has a residual compressive stress of about 165 ksi,and can withstand regular stress of 137.5+165=302.5 ksi and stillsurvive one million cycles. If we then let S=302,500 in equation (1),and solve for t, we find that the wire can thickness can be 0.030inches. This translates to a weight reduction of 41% compared to a nonshot-peened shoe, and a reduction of about 1.64 ounces per shoe. In,e.g., a typical running shoe, a reduction in weight by 1.64 ounces canbe the difference between a satisfactory and unsatisfactory weight.

In preliminary tests, we found that running shoes with shot peened,crest-to-crest wave springs last at least ten times longer than shoeswith comparable non-shot peened crest-to-crest wave springs. Forexample, in one test, non-shot peened springs failed after approximately50 miles of running, while shot-peened springs remained functional after500 miles.

Other Embodiments: Other embodiments are within the spirit and scope ofthe invention. For example, the shot peening process can be modified. Inthe above described embodiment, the springs were shot peened for 10-15minutes, until 100% of the surface was covered (i.e., the dimples 5overlapped). Alternatively, the shot peening can continue for twice theamount of time needed for 100% coverage, such that the surface is “200%”covered with dimples. In addition, less of the surface can be shotpeened, e.g., 10%-50%, 50%-100% or 100%-200%. Different sized anddifferent shaped shot can also be used.

After shot peening, the springs can be baked at, e.g., 205 degreesCelsius for, e.g., about 30 minutes, to reduce the likelihood ofsetting. Other times and temperatures are possible, so long as thetemperature is not so high that it relieves the residual compressivestress imparted by the shot peening. (See Metal Improvement Company,Inc., Shot Peening Applications 25 (8th ed. 2001).

In the above described embodiment, all exposed surfaces of thecrest-to-crest wave springs are shot peened. It is also possible to shotpeen the non-exposed surfaces where crests from different turns contacteach other. This can be done by stretching the spring to pull thedifferent turns apart during the shot peening process, to expose thecrest surfaces which contact each other when the spring is relaxed.

Springs other than wave springs can be shot peened and placed withinshoes. For example, it is possible to shot peen coil springs, disksprings, Belleville springs, spiral springs, cone springs, or othertypes of springs, and then locate them in the sole of a shoe. The shotpeened springs can be placed within heel and ball vacuities of the shoebetween an inner sole and an outer sole, as described in U.S. Pat. No.6,282,814, or can be placed only within the heel area, or in other areasof the shoe sole. Alternatively, the spring may be mounted in a shoehaving an inner sole and an outer sole, but no side walls.

The springs can be treated with other processes which impart a residualcompressive stress instead of, or in addition to, shot peening. Forexample, the springs can be treated with roll burnishing, knurling,compression over a mandrel, heat treating, and “magnaforming” (largemagnets impart compressive stress).

While the present invention has been illustrated by description ofseveral embodiments and while the illustrative embodiment has beendescribed in considerable detail, it is not the intention of theapplicant to restrict or in any way limit the scope of the appendedclaims to such detail. Additional advantages and modifications willreadily appear to those skilled in the art. The invention in its broaderaspects is therefore not limited to the specific details, representativeapparatus and methods, and illustrative examples shown and described.Accordingly, departures may be made from such details without departingfrom the spirit or scope of applicant's general inventive concept.

Having thus described the aforementioned invention, we claim:

1. A process for reducing heat transfer within a spring in a springcushioned shoe, said process comprising the steps of: applying aresidual compressive stress to said spring; and mounting said springbetween an inner sole and an outer sole of said shoe.
 2. The process ofclaim 1 wherein said step of applying a residual compressive stress isselected from a group comprising shot peening, roll burnishing,knurling, compression over a mandrel, heat treating and magnaforming. 3.The process of claim 1 wherein said spring comprises a multi-turncrest-to-crest wave spring.
 4. The process of claim 1 and furthercomprising the step of baking said spring after applying a residualcompressive stress to said spring.
 5. The process of claim 1 wherein thecompressive stress applied to said spring is about 50% of the tensilestrength of said spring.
 6. The process of claim 1 wherein said step ofapplying a residual compressive stress comprises shot peening.
 7. Theprocess of claim 6 wherein said shot peening step creates dimples over10 to 200% of the surface of said spring.
 8. The process of claim 6wherein said shot peening step creates dimples over 50 to 100% of thesurface of said spring.
 9. The process of claim 6 wherein said spring isstretched during said shot peening to expose additional surfaces forshot peening.